This invention relates to detectors and, in particular, to detectors fog gamma radiation.
1. FIELD OF THE INVENTION
2. DESCRIPTION OF RELATED ART
Referring now to the drawings, FIG. 1 shows a prior art design. Incident gamma radiation causes a BaF.sub.2 crystal 1 to scintillate, generating ultra-violet (uv) photons. The UV photons convert in a gas space 3 adjacent to the crystal and the resulting handful of electrons are amplified in a high electric field applied between a conductive mesh 5 on the crystal surface and the cathode 7 of a multiwire proportional counter (MWPC) 9. The signal is transferred into this section and further amplified on the anode wires 11. Some form of read-out is built into the MWPC section.
We found that this structure was very unstable and started to breakdown after 20 minutes or so, due to the charging of the crystal surface by the positive, ions returning from the avalanche in the MWPC. However, we attempted to address this instability by installing a protective gap 13 (FIG. 2) with a reverse bias immediately against the crystal face. This gap (preferably 0.5-1.0 mm wide) sacrifices a little signal for a very much enhanced stability. He have found that with 100 V of reverse bias the modified counter will run all day without showing charging effects.
In the prior art positron camera a severe practical problem is caused by the very high ratio of single counts to coincidence (i.e. useful) counts (up to .apprxeq.50:1). This overloads the gain elements of the MWPC and causes serious deadtime losses in the read-out system. He found two further modifications which improve this situation significantly. Firstly, the initial parallel amplifying gap 3 which now follows the crystal barrier gap is separated from the MWPC by a wide gap 19 (.about.30 mm). (This on its own further enhances the stability of the counter.) In order to do our fast coincidence we would now like to take a trigger signal from the back of this gap. However, as this would demand too much gain from one gap we insert a further gap 21 and take the trigger signal from its rear face.
Secondly the fast coincidence with the other detector is performed while the electron cloud drifts towards a MWPC 9 which delivers a final burst of gain and performs the read-out. Roughly in the middle of the drift region 15 is an electronic gate 17 operated by the coincidence circuit. This ensures that only "good" events trigger the MWPC and the read-out system. This simultaneously enhances the stability of the counter and dramatically reduces the pile-up in the read-out electronics. This gate has been carefully designed with shield electrodes 23,25 to minimise the interference it can cause in the read-out electronics.
With these modifications our tests to date have been able to demonstrate a quantum efficiency of 20% and a spatial resolution of 6 mm fwhm with a time resolution of 4 ns fwhm. The efficiency is three times that of the lead system and the time resolution 1/3. This means a factor of 9 in sensitivity and a factor of 27 in signal to noise ratio. The predicted maximum data rate rise is from 2 kHz to 20 kHz under comparable conditions.